In general, a camera includes a lens for capturing (forming) an image of an object or subject onto an image sensor, and an image sensor for detecting the image formed by the lens as an electric signal. A focal point of the lens changes depending on a distance between the lens and the object. An image with superior quality is able to be captured only when a variation of a position of an image surface based on a position of the object is within a range of a focal depth (depth of focus) of the camera. Hence, a typical camera requires for a function for automatically adjusting a focal point according to the change in a distance from an object, namely, an autofocus (AF) function, to obtain an image with excellent quality.
Most of digital cameras use a through-the-lens (TTL) contrast detection. The contrast detection is to continuously calculate a contrast of a part (usually, a central portion) of an image while moving a lens, and determine to be focused when the highest contrast is calculated. Therefore, an autofocus function aims to obtaining the sharpest image at a lens step with the highest focal value by analyzing lens steps, which are set according to a distance between the lens and an object or subject and are units for moving a position of the lens, and focal values, each of which is converted into a numeral value from high frequency image signal information, for example, based on characteristic information (for example, sharpness information) relating to an image which is formed on an image sensor at each lens step.
A representative of the autofocus algorithm is hill climbing. The hill climbing type autofocus algorithm is a method of moving a lens from a current position to front and rear sides by a unit of a lens step and thereafter moving a focal lens in a direction to be focused so as to find an optimal focal position.
To focus on an object according to the hill climbing method, the focal lens is moved by each lens step to detect focal values, and then is located on the lens step with the highest focal value. Accordingly, an autofocus time increases as long as the number of steps. This may cause a delay by the autofocus execution time during capturing of an image, resulting in impossibility of obtaining a sharp image for the object in the right place at the right time.